It's the health message that comes around every year with the first warm day: beware of the sun. The dangers of skin cancer from unprotected sun exposure are now well known. But that's only half the story.
The health value of sunlight, lauded by the ancient Greeks and Romans, has been largely eclipsed in recent years by concerns about skin cancer. But while this is a very real, and increasing, problem - more than 7,000 people are diagnosed with malignant melanoma in the UK each year - evidence is also emerging that small to moderate amounts of exposure, like modest amounts of red wine, may be positively healthy.
IT CAN PROTECT AGAINST CANCER
Sunlight plays a vital role in the production of vitamin D in the body, and it's believed that the vitamin may have a role in stopping or slowing the growth of tumours by preventing the overproduction of cells, as well as in boosting bones. Vitamin D is available in some foods, but it is estimated that up to 90 per cent comes from exposure to sunlight.
"Vitamin D sufficiency is required for optimal health. The conditions with strong evidence for a protective effect of vitamin D include several bone diseases, muscle weakness, more than a dozen types of internal cancers, multiple sclerosis and type-1 diabetes mellitus,'' says Dr Michael Holick of Boston University School of Medicine.
One of the first clues to a possible beneficial link between cancer and sunlight was the discovery of large geographic differences in the prevalence of colon cancer deaths in America. The rates in the states in the north were treble those in the south.
Since then, researchers have found that incidences of other cancers also vary according to levels of sunlight.
Researchers at the Naval Health Research Center in San Diego found that the right levels of vitamin D halve the risk of colon cancer. "Prompt public health action is needed to increase intake of vitamin D by encouraging a modest duration of sunlight exposure,'' they say.
Increased exposure to sunlight may also decrease the risk of prostate cancer, according to the findings of a research team from three cancer centres, including the Northern California Cancer Center. Researchers found that prostate cancer risk for men with high sun exposure was half that of men with low sun exposure.
IT CAN BOOST SURVIVAL RATES
Research based on more than one million cancer patients in the UK shows that those diagnosed and treated in the summer and autumn are likely to survive longer compared with those diagnosed in the winter. For women diagnosed with breast cancer, survival chances increased by 14 per cent, and for men and women with lung cancer, there was a five per cent better survival rate.
Researchers looked at season of diagnosis and survival rates of people registered with cancer between 1971 and 2002. In total, 588,435 men and 606,127 women from the Thames Cancer Registry were included in the analysis. "We found substantial seasonality in cancer survival, with diagnosis in the summer and autumn months being associated with improved survival, especially in lung and breast cancer patients,'' say the researchers. "Our results add to a growing body of evidence that vitamin D may play an important role in cancer survival."
They add: "Cumulative sunlight exposure in the months preceding diagnosis was also a predictor of subsequent survival.''
Dr Lesley Walker, the director of cancer information at Cancer Research UK, says that it is not clear whether the effect is down to sunlight exposure or simply seasonal variations in mortality: "The relationship between vitamin D and the development of cancer is complex and remains unclear,'' she says.
IT'S GOOD FOR YOUR BONES
Take a walk on a rainy or cloudy day to lower the risk of osteoporosis, while avoiding the risk of skin cancer.
Vitamin D deficiency is linked to osteoporosis and bone health and growth, and may contribute to bone fractures. Low levels of the vitamin have also been associated with rickets. According to research at the Radiation Effects Research Foundation, Japan, 100 to 200 units of vitamin D need to be obtained each day from sunlight: "Considering the risks of skin cancer caused by sunlight exposure, short trips outside on sunny days, and 30-60 minutes on rainy days, would be appropriate,'' they say.
Vitamin D helps to increase the absorption of calcium and phosphorus from the small intestine, regulate the amount of calcium in the blood, and strengthen bones and teeth. A deficiency of vitamin D leads to a failure of the bones to grow and causes rickets in children and osteomalacia in adults.
In Australia, where there has been considerable health education targeting of skin cancer, new calcium, vitamin D and osteoporosis guides recommend increasing calcium intake and exposure to limited sunlight in order to reduce the risk of developing osteoporosis. "Vitamin D deficiency can cause rickets in children, resulting in muscle weakness and bone deformities. The national 'no hat, no play' policy in school playgrounds should be reassessed in Canberra and Tasmania during winter, for instance, because children aren't being exposed to sufficient sunlight," says Judy Stenmark, chief executive of Osteoporosis Australia.
"While vitamin D is found in small quantities in a few foods such as fatty fish, liver, margarine and eggs, for most Australians, especially children, adequate vitamin D is unlikely to be achieved through diet alone."
IT SOOTHES PAIN
When doctors carried out a review comparing the outcomes of patients recovering from a spinal injury or heart attacks, they found that people whose beds were on the sunny side of the hospital seemed to have less pain than those whose rooms were in the shade.
The spinal surgery team from the University of Pittsburgh found that the difference could not be explained by variations between patients or in treatments they had received. What was making the difference, they discovered, was the sun. They found that patients on the bright side of the hospital unit were exposed to sunlight that was 46 per cent more intense. Patients of all ages in sunny rooms had less pain, used 22 per cent fewer painkillers, and were less stressed.
Similar findings have been found for heart attack patients, but just how sunshine works in pain control is not clear. One theory is that the feel-good brain chemical serotonin, boosted by exposure to sunlight, is involved. Another suggestion is that other brain chemicals, or even the warmth of the sun, may have a beneficial effect.
IT CAN SPEED RECOVERY
Patients who want to improve their chances of survival should insist on a bed in a hospital ward that catches the sun.
Researchers have discovered that, as well as having less pain, patients in beds exposed to high levels of sunlight get healthier sooner, go home up to four days earlier, need fewer drugs and are less likely to die prematurely.
IT PREVENTS DEPRESSION
The finding that one in four people living in Alaska suffered with depression was one of the first clues of a link between sunlight and mood. That and other research led to the theory that seasonal affective disorder, or SAD, increases in the higher latitudes where the winter days are short - four hours or less in Alaska.
Since then, research has gone on to show links with other forms of depression. At the University of Milan, doctors found that patients admitted with clinical depression who were allocated hospital beds with high levels of sunlight in the mornings went home 3.67 days earlier on average. "Natural sunlight can be an underestimated and uncontrolled light therapy for bipolar depression,'' say the researchers.
Serotonin is thought to be involved. Most antidepressant drugs work on controlling levels of this chemical, and research shows that levels increase in relationship to sun exposure.
IT CAN PROTECT AGAINST MULTIPLE SCLEROSIS
Sunlight may protect against MS and explain why the disease is more prevalent in areas most distant from the equator. Studies show that the disease, whose cause is unknown, is also more common at low altitudes than at high altitudes, where the intensity of ultraviolet radiation is much stronger.
Research at the University of Tasmania found that higher sun exposure between the ages of six and 15 - an average or two to three hours or more a day in summer during weekends and holidays - more than halved the risk of getting the disease. Winter sunshine was particularly beneficial.
One theory is that exposure to sunlight boosts the immune system to prevent the damage involved in the disease. Another is that vitamin D plays a key role in the growth of the developing brain.
IT KEEPS US HEALTHY
The risk of myriad diseases and disorders can be reduced by sunlight, according to researchers around the world. "Epidemiological data indicate a low vitamin D status in tuberculosis, rheumatoid arthritis, multiple sclerosis, inflammatory bowel diseases, hypertension, and specific types of cancer,'' says a team from the University of Bonn.
"Sunlight can have beneficial effects, and may protect against auto-immune diseases including type I diabetes and multiple sclerosis," say researchers from the University of Sussex.
"Evidence suggests that ultraviolet radiation may play a protective role in three auto-immune diseases: multiple sclerosis, insulin-dependent diabetes mellitus and rheumatoid arthritis,'' says the Australian National University.
IT CAN HAVE AN EFFECT ON FERTILITY
According to research from Israel, solar and cosmic ray activity affects the numbers of babies coming to term, andthe gender balance of births. Based on a study of 286,963 babies in Lithuania, this research suggests that high solar activity reduces the chances of a pregnancy developing. The results also point to more boys being conceived during high solar activity.
It isn't clear why this happens: "The monthly number of newborns of both genders is strongly and significantly related to the level of monthly cosmic ray and, inverse, to solar activity indices nine months before delivery,'' say the researchers.
You still have to be careful...
* More than 65,000 cases of skin cancer are diagnosed each year in the UK, making it the most common type of cancer. The real number of cases may be much higher because of under-reporting.
* Most cases of skin cancer are easily treated and cured. Cancer Research UK says that many could be prevented if we were more careful about exposing skin to the sun.
* Each year, about 8,000 people are diagnosed with melanoma, potentially a more serious type of skin cancer. Many cases could be prevented with care about sun exposure, and if people were more thorough in looking for signs that it might be beginning to develop, including a mole that gets bigger, or one that changes shape or becomes itchy or painful.
SunSmart exposure advice from Cancer Research UK:
* Stay in the shade 11am-3pm
* Make sure you never burn
* Always cover up
* Take extra care with children
* Use factor 15+ sunscreen
For more information visit www.cancerresearchuk.org/sunsmart/